In certain industrial processes, it is necessary to add lithium to a substrate. In particular, electrochromic devices, which are adapted to change optical properties in response to changes in an applied electrical potential typically include a plurality of layers incorporating mobile lithium ions. Under the influence of an applied potential, the lithium ions will migrate from one layer to another. The various layers are selected so that the optical properties change depending upon the concentration of lithium in each layer. Materials of this nature are disclosed, for example, in U.S. Pat. No. 5,370,775. These materials can be used in optoelectronic devices such as light modulators, display devices and the like. Electrochromic materials can also be used in selectively controllable window systems for various applications, including windows on buildings and vehicles. Certain production processes for making electrochromic materials require application of lithium to the electrochromic material after the same is formed. As disclosed in the '755 patent, this can be accomplished by exposing the electrochromic materials to an electrolytic process using an electrolyte bearing lithium ions. Although this process is effective, it requires exposure of the substrate bearing the electrochromic layer to a liquid electrolyte. This, in turn, can add to the cost of handling substrates, particularly large substrates such as window glass panes.
It has been proposed heretofore to use sputtering to apply lithium to a substrate such as an electrochromic substrate. In the sputtering, ions are impelled against an exposed surface of a source or "target" formed from the material to be applied, as by imposing an electrical potential between the target and a counterelectrode while maintaining the target in proximity to the substrate. The energetic ions impacting on the target dislodge atoms of the target, commonly referred to as "adatoms", which then deposit on the substrate. Typically, such a process is conducted in a gaseous atmosphere maintained under a very low subatmospheric pressure. The gaseous atmosphere is ionized to form a plasma, a mixture of ionized gas atoms and free electrons. Ions of the gas form the energetic ions which bombard the target. The potential applied between the target and the counterelectrode ordinarily is a fixed (DC) potential, wherein the target is negative with respect to the counterelectrode, where the target is a conductive material. An alternating potential at radio frequencies (RF) typically is used when the target is a dielectric material. Most commonly, the radio frequencies used for such sputtering are at the particular radio frequencies reserved by communications authorities for industrial, scientific and medical uses, the so-called "ISM" frequencies, most typically about 13.56 MHZ or higher.
Targets formed from lithium compounds such as Li.sub.2 CO.sub.3 can be successfully sputtered to deposit lithium into electrochromic materials. In large scale systems, however, the RF sputtering potential required with a Li.sub.2 CO.sub.3 target presents process problems such as nonuniformity and requires expensive equipment for generating and handling high power RF. It would be desirable to use a sputtering target having an exposed surface consisting essentially of pure, metallic lithium. Such a metallic lithium sputtering target at least in theory should provide faster more uniform deposition of lithium into the substrate particularly in a relatively large-scale process. As set forth in U.S. Pat. No. 5,288,381, proposals for use of a lithium metal target surface have been advanced. However, there has been no practical process heretofore for sputtering lithium from a target having a metallic lithium surface. In particular, it has been impractical to sputter lithium at a reasonably fast rate from a target having metallic lithium at its exposed surface using DC sputtering potential without damaging the target. It has also been difficult to fabricate lithium sputtering targets heretofore.
There have, accordingly, been substantial unmet needs for further improvements in lithium sputtering processes. There have been further needs for improvements in sputtering targets for use in such processes and in methods of making such targets.